The present invention relates to methods and devices for retrofitting large steam turbines, particularly low pressure steam turbines and even more specifically the last or next to last stage of low pressure steam turbines.
In the following description the term “turbine” is used to refer to rotary engines having a rotating part and a stator part force coupled by a fluid medium such as water, steam or gas. Of particular interest for the present invention are axial steam turbines comprising radially arranged fixed stator blades or vanes alternating with radially arrangements of moving rotor blades force-coupled by a flow of steam through the turbine. Movements are generally defined as movements relative to a casing or housing.
In large turbines, particularly steam turbines, the moving blades or airfoils are presently manufactured using steel or titanium based alloys. In a multi-stage turbine, the size of the blades increases from stage to stage. In the final stage of the largest low pressure turbines the height of a turbine blade can exceed one meter. While it is desirable to increase the size of the turbine stages and thereby increase their exhaust surface and efficiency, the properties of current materials have reached theirs limits mainly because of the large centrifugal forces acting on the rotating blades.
To overcome the barriers set by the materials properties of steel and titanium, composite material airfoils have been proposed using mainly carbon fiber based materials. Though a large number of such designs has been published, real-world applications of such composite blades are currently limited to gas turbines for advanced aircrafts engines.
The moving blades are usually mounted onto the turbine rotor using root sections on the blades, for example the so-called “fir trees”, which mechanically lock into slots cut into disks on the rotor. However the optimal root for an airfoil made of fiber-reinforced composite material is typically different from the root sections of conventional metal airfoils. This difference makes it difficult to replace metal airfoils by composite airfoils when repairing, upgrading or retrofitting an existing turbine which was originally designed for metal blades.
U.S. Pat. No. 3,883,267 proposes a blade for a fluid dynamic machine with an air-foil section constituted by the superimposition upon a metal core, of a plurality of superimposed layers of composite fibrous material, said metal core having a portion which projects beyond said airfoil section in order to act as a blade attachment root.
In view of the known prior art, it is seen as an object of the invention to provide methods and devices which facilitate the retrofitting of large turbines with airfoils or blades of advanced materials such as composite fiber materials.